The present invention generally relates to optical control devices formed on a photonic crystal. More specifically, the present invention relates to a compact and high-performance optical control device for use in the field of optical communication such as high-speed and large-capacity optical signal transmission or high-speed optical signal processing. Further, the present invention relates to an optical control device capable of realizing apparatuses and devices such as compact optical pulse delay devices providing a large delay in the group velocity, dispersion compensation devices providing large dispersion compensation effect, non-linear optical devices providing high efficiency, lasers operating with high efficiency, optical routing devices and advanced optical information processing apparatuses, optical buffer devices, and the like.
In the art of high-speed and large capacity optical communication or high-speed optical signal processing, the phenomenon of dispersion, which induces decay in the optical pulses transmitted along an optical fiber, or skew, which causes a change in the arrival time of optical signals transmitted along an optical fiber, poses a serious problem to be overcome in order to achieve further increase of transmission speed.
In order to solve these problems, there is a need of a device capable of controlling the velocity of optical energy, which determines the dispersion characteristic or signal arrival time, while this means that there is a need for a device capable of controlling the amount of delay in the group velocity of optical pulses.
Conventionally, such delay of optical pulses in terms of group velocity has been controlled by using an optical fiber having a singular dispersion characteristic. According to this approach, the length of the optical fiber used for transmitting the optical signals is adjusted such that there is realized an optimum amount of delay in terms of the group velocity for the optical signals transmitted along the optical fiber.
However, because of small dispersion caused by such an optical fiber, there is a need of using a long optical fiber for achieving the desired control of delay of the optical signals, and there arises a problem that the optical control device inevitably has a large size even in the case the optical fiber is coiled to reduce the size thereof. Further, because of the small degree of freedom in the dispersion characteristics of the optical fiber, it is not possible with this approach to achieve downsizing or integration, which is necessary for realizing advanced signal processing, or parallel signal processing that includes a number of transmission paths.
Further, with regard to the compensation of dispersion, this conventional technology enables precise dispersion control or adjustment of dispersion compensation by using a chirped fiber grating technology, in which there is formed a grating in the optical fiber such that the period of the grating is changed gradually.
However, because of the small dispersion provided by the optical fiber, it is necessary with this approach to use a long optical fiber in the order of meters for achieving the desired compensation effect of dispersion, and thus, it is not possible to achieve downsizing or integration for optical control devices.
Further, with the technology of such a fiber grating device having the chirp structure, a reflected light is used in addition to the incoming light, and there is a need of providing a structure for separating the incoming signals and outgoing signals for efficient operation. This also poses an adversary problem with regard to downsizing and integration of the optical control device.
As an alternative of realizing low optical group velocity, there is known an approach of confining the light by using a multilayer film in the form of optical multiple reflection. However, such a construction of using multilayer film for achieving low optical group velocity or dispersion control has a problem, associated with small effect of optical confinement of the multilayer film, in that the size of the device becomes inevitably large, and the device suffers from the problem of spreading of the optical signals by diffraction. Thus, it is difficult with this approach to control the dispersion as desired.
In view of these problems, Patent Reference 1 and Patent Reference 2 disclose a dispersion compensation device that uses a photonic crystal, wherein a photonic crystal is a multi-dimensional periodic structure formed by different refractive indices.
More specifically, the wavelength dispersion compensation device of Patent Reference 1 has a construction of injecting an incident optical pulse having a wavelength-dispersion and hence an associated chirp into an edge surface of a photonic crystal in which media of different refractive indices are arranged in the form of two-dimensional lattice.
The optical pulse thus injected undergoes a decrease of chirp as it is transmitted through the photonic crystal as a result of the dispersion characteristic of the photonic crystal.
Further, the wavelength dispersion device of Patent Reference 2 compensates for the wavelength dispersion by utilizing the dispersion characteristics of light that is guided along an optical waveguide, which is formed in the photonic crystal in the form of defect.
In a photonic crystal per se, or in an optical waveguide called defect waveguide, which is formed in a photonic crystal by introducing a line-shaped defect thereto, there appears a singular dispersion characteristic, which describes the relationship between frequency and wavenumber.
On the other hand, with the wavelength dispersion device of Patent Reference 1, in which the transmitted light is not confined into a waveguide structure in the photonic crystal, there appears a problem of poor reliability associated with its large angular dependence. Further, the device of this reference is deemed not practical in view of difficulty of achieving downsizing.
On the other hand, it is theoretically predicted that the group velocity should become zero in a line-defect waveguide at the Brillouin zone edge called also band edge. It should be noted that a line-defect waveguide is a waveguide formed in a photonic crystal in the form of a continuous line-defect. Patent Reference 1 reports observation of a very small group velocity of 1/90 the velocity of light in vacuum.
On the other hand, such a line-defect waveguide is generally accompanied with a very large wavelength dispersion, and because of this, while it is certainly possible to decrease the group velocity when a short optical pulse having a spread spectrum is injected into such a structure, there arises an adversary problem of dispersion in that the optical pulse undergoes excessive spreading because of the spreading of the spectrum width.
Further, with the structure of Patent Reference 2 called coupled-defect waveguide, in which point-shaped defects are arranged periodically, a relatively large dispersion is achieved over a relatively wide band width. Because the value of dispersion is larger than the dispersion in an optical fiber by the order of six (106), there is a possibility that a fiber dispersion compensation device, which has needed the size of the order of kilometers, is subjected to downsizing to the size of millimeters.
However, when such a coupled-defect waveguide is formed in a slab-formed photonic crystal, which can be produced relatively easily, there appears a fundamental problem in that the light in the photonic crystal is scattered in the direction perpendicular to the surface on which the photonic crystal is formed because of diffraction caused by the photonic crystal, in view of the fact that the period of repetition is increased in the propagating direction of the light. Thereby, there is caused a problem of very large optical loss.